A tumor-host system with unique features of metastasis in puppies but with spontaneous regression after 2 to 5 months of rapid growth in adult non-preconditioned dogs will be used to evaluate the role of various sub-populations of mononuclear cells infiltrating the tumor mass at different stages of growth. Special emphasis will be placed on the identity of the cells responsible for spontaneous rejection of the tumor. The rejection process, once started, generally completes within 2 to 3 weeks and an understanding of the mechanisms responsible for this phenomenon which occurs in both nature and in the laboratory, is of utmost importance in tumor and transplantation immunity. In addition, the mechanism of materno-offspring transfer of tumor immunity previously observed in this laboratory will be studied.